Design and Analysis of Brushless Self-Excited Three …12/793-oliveira.pdfsystem, half-wave...
Transcript of Design and Analysis of Brushless Self-Excited Three …12/793-oliveira.pdfsystem, half-wave...
European Association for the
Development of Renewable Energies, Environment
and Power Quality (EA4EPQ)
International Conference on Renewable Energies and Power Quality
(ICREPQ’12)
Santiago de Compostela (Spain), 28th to 30th March, 2012
Design and Analysis of Brushless Self-Excited Three-Phase Synchronous Generator
M. O. Oliveira
1-2, A. S. Bretas
1, F. H. García
1, L. A. Walantus
2, H. E. Muñoz
2, O. E. Perrone
2 and J. H. Reversat
2
1 Electrical Engineering Department
UFRGS, Federal University of Rio Grande do Sul
103 Osvaldo Aranha Avenue, Porto Alegre-RS (Brazil)
Phone/Fax number: +55 51 3308-4437, e-mails: [email protected], [email protected], [email protected]
2 Energy Study Center to Development –CEED
UNaM, National University of Misiones
327 Juan Manuel de Rosas Street, Oberá-Mnes. (Argentina)
Phone/Fax number: +54 03755-422170, e-mails: [email protected], [email protected],
[email protected], [email protected]
Abstract. This paper presents the main features of design, construction,
performance analyses and experimental studies of a brushless
self-excited three-phase synchronous generator. The basic
construction, principle of operation, and exciting characteristics
are described. In proposed machine the rotor winding is shorted
through diodes and the self-excitation is reached by a slip
between rotor and stator winding built with different pole
numbers. Computer simulations performed with MATLAB®
software were used to verify the mathematical model of the
brushless self-excited synchronous generator and the achieved
results showed similar behavior with experimental values.
Key words
Brushless generator, synchronous machine, self-excited
system, half-wave rectified, excited capacitor.
1. Introduction
Synchronous generators and the associated control system
constitute one the most important and complex projects
presented in the electric power system. These machines
generate electricity which is then transported to
consumption center through transmission lines. This
generators type is typically projected with the slip rings
whose terminals can be used with a steady source power
for excitation. The carbon brushes are the most critical
parts of the power generator because they demand an
important maintenance level, and therefore are preferred
than the brushless generators to supply energy to critical
loads. The brushless generators usually consist of a
permanent magnet to eliminate rotating excitation
winding, carbon brushes and slip rings [1].
On the other hand, the small capacity generator works in
severe operation conditions, therefore, it is essential that its
construction is simpler and robust. Also, self-excitation
and self-regulation of voltage in a synchronous generator
is important to reduce the maintenance level and the
control system complexity in case of large load
variations.
In this sense, many project and studies of synchronous
generators have been proposed in the last years. Nonaka
and Kesamaru [2] described a new brushless generator
without exciter rotating where the stator has two
independent windings with different pole numbers. The
first is the single-phase load winding and the second is
the exciter winding in direct current (DC). This generator
has the advantage of presenting an almost constant output
voltage for large load variations and different speeds. On
the same theme, Inoue et al. [3] proposes a novel
brushless generator with three-phase armature windings
on the stator, one field winding and one exciting winding
with five time as many poles of the armature winding on
the rotor, and a three-phase reactor connected to the
terminal of the armature winding. The disadvantage is the
need of an external battery since the capacitors system is
not fully efficient. More recently, Chan and Lai analyzed
the steady state behavior of a single-phase self-excited
generator where the operation equations are derived
using the symmetrical component method [4]. Hooke and
Jeeves method is used to determine the machine
variables. A more detailed discussion of some excitation
control systems for brushless synchronous generators is
presented in [5]-[7], and different methods of voltage
regulation in generators are proposed in [8]-[10].
In this context, the objective of this paper is to present
and discuss the results obtained from the modeling,
construction and testing of a synchronous generator with
asynchronous excitation (SGEA) and with an
uncontrolled rectified bridge. On the last, the terminal
voltage of the machine is regulated through the field
current. The test results obtained with the prototype test
are acceptable and encouraging.
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2. Modelling of the Brushless Self-Excited
Synchronous Generator
Although the field winding of a traditional synchronous
generator is in the rotor and the field winding of a
homopolar generator is located in the stator, both machines
share the same terminal characteristic and can be described
with the same parameters grouped [1]. The classic model
of separately excited synchronous machines can be
represented in rotational coordinate d-q [10]. However,
detailed modeling of synchronous generator for transient
studies should include a variable phase model, orthogonal
d-q axis model and finite element analysis [9], [2].
Fig. 1 shows the basic circuit of the brushless self-excited
synchronous generator designed in this work. The stator of
this machine has two windings, the load winding Wa and
the auxiliary self-excitation winding Wc, connected to a
variable capacitor C. The rotor has only one field winding
Wfd, shorted with diodes D. For a given output frequency f,
a voltage is induced on the Wfd by the reverse field due to
the armature reaction. This voltage is then rectified in half-
wave to obtain the excitation voltage of the synchronous
generator.
Fig. 1. Electric circuit for brushless self-exciting three-phase
synchronous generator.
The basic steady-state equations for modeling the
brushless self-excited synchronous generator were
obtained from [11]. The following simplifying
assumptions were considered:
• Negligible winding mmf (magnetomotive force) and
saliency harmonics.
• Rotor cage equivalent to sinusoidally distributed
winding on each axis.
• Constant rotor speed.
• Negligible rotor field winding resistance so that field
flux is considered constant.
Other important aspects for modeling the brushless
self-excited synchronous generator are presented in [12]-
[15].
A. D-q Model for Single Phase Synchronous Machine
Steady state armature current is given by the following
equation [11]:
1
cos( )a an r an
n
I I n tω∞
=
= + Φ∑ (1)
and the exciter winding current has the form:
1
cos( )b bn r bn
n
I I n tω∞
=
′ ′= + Φ∑ (2)
where Ian, Φan, I’an, and Φbn are unknow.
The temporal and spatial variation of flux linkage
between the exciter winding and armature is defined by
the following expression:
[ ]cos( ) ( )
( ) cos( )
a r r
b r r
t sen tM
sen t t
ψ ω δ ω δ
ψ ω δ ω δ
+ + = × ′ − + +
(3)
where the M matrix is defined as:
0 00
00
r
qmq
rbq rq
dr
d md mdd
frb fr
dIZ dt
x I dI
x Z x dtI
x
ω
ψω
′′
− + ′′ ′ ′ ′
(4)
where the parameters xq, xd, Z’’mq, Z’’md, x’md y x’fr were
calculated as detailed in [11].
The armature winding flux linkage, Ψa, from (3) is
substituted into the armature voltage expression:
0aa
b
dR I
dt
ψ
ω+ = (5)
where R is the load resistance.
The exciter winding flux linkage, Ψ’b, is substituted into
the exciter voltage expression:
0blbs b b b b c b
b
d dx I r I X I dt
dt dt
ψω
ω
′′ ′ ′ ′ ′+ + + =∫ (6)
where X’c is the capacitive reactance referred to the
armature winding.
According to [11], the resulting equations consist of
equations of fundamental frequency and an infinite series
of harmonics voltages. The equation of excitation current
and armature are given by the following expressions:
1 1( )
1 1bj j
b b frI e K eδ γΦ +′ ′= Ψ (7)
1
( )
3 1
n
k
bn k
j nj n
bn k bn bI e K I e
δ φ
π =
+Φ
=
∑′ = (8)
1 1 1( )
1 1 1a bj j
a a bI e K I e
αΦ Φ +′= (9)
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( )
3 1an bn nj jn
an an k bk bI e K K I e
απΦ Φ +
== (10)
where Kbn, γn, Kan and αn are functions of the machine
parameters. The load angle, δ, and field flux, Ψ’fr, are
unknowns.
Thus, if the field winding resistance is neglected then and
assuming the field diode commutates when time t=0, it is
possible to obtain:
00
rfr rmd md d
fr dtfr md lfr md
x Z dII I
x x x x dt=
′Ψ ′′′ = − + =
′ ′ ′+ (11)
2
2 200
r r
md d md dfr t
b b
x dI Z d Ie
dt dtω ω=
′′′ = − − = (12)
2 3
2 3 3
0
0r r
fr md d md d
b bt
de x d I Z d I
dt dt dtω ω=
′ ′′= − − ≥ (13)
where I’fr is the field current and e’fr is the field voltage
applied across the field diode.
3. Electric Circuit and Mechanical Structure
of the Synchronous Generator Proposed
The proposed SGEA was built using the mechanical
structure of an asynchronous motor of 5HP, 1000 rpm.
This generator is asynchronously self-excited, ie, the
rectified alternating voltage used in the generator
excitation is obtained through a slip between the rotor and
stator windings. The stator consists of an auxiliary field
winding in parallel with a capacitor bank and a stator
winding load. The voltage induced in the rotor field
winding is one-phase and half-wave rectified.
A. Operating Principle
Fig. 2 shows the configuration and arrangement of the
rotor and stator windings of the proposed synchronous
machine.
Fig. 2. Stator and rotor winding single-phase configuration.
When the generator starts to turn, it is induced on the
single-phase winding an electro-motive force (fem) due to
the residual magnetism of the rotor. Thus, a current IC will
start flowing in the circuit formed by the parallel between
single-phase winding and the capacitor C connected to it.
The IC current, known as magnetizing current of self-
excitation, is reinforced by the capacitor current once the
generator reaches steady state. It is important to recall
that a difference exists between the variables related to
magnetic fields due to the main field speed (4 poles) is
1500 rpm and auxiliary field speed (6 poles) is 1000 rpm.
The generator rotation speed is kept stable at 1500 rpm so
there is a difference in favor of the excitation current IC.
The single-phase winding of six poles takes the capacitor
the reactive power for excitation and the capacitor size is
determined in this design. The field excitation current of
the stator (IC) produces a magnetic field B through the air
gap and induces on the rotor winding a new fem
responsible for creating the main excitation field.
B. Stator Circuit Characteristics
The stator consists of the auxiliary and load winding.
Both windings have a sinusoidal distribution of magnetic
flux in the air gap produced by the stator current of the
machine. In this circuit, it is very important the
orientation of the magnetic axis between the two
windings for the machine work correctly. In this sense,
the angle between the magnetic axis of the load winding
(power coil) and auxiliary winding (connected to the
capacitor bank) should be 90 electrical degrees.
C. Rotor Circuit Characteristics
The most appropriated is to make the rotor magnetic flux
a uniform distribution along the air gap and a magnetic
density B with constant geometric distribution. Thus, as
the rotor rotates at a constant speed, the magnetic flux in
the generator varies in pulses form with a frequency
twice the stator frequency. This variation causes that the
turns “see” (or be cut) by a relative flux that varies
sinusoidally in space and time.
In practice, the rotor current is not exactly pulsing and
sinusoidal. This problem must be compensated by the
rotor magnetic circuit shape, for example, changing the
polar heads or decreasing the air gap. This situation
occurs with the deformation of the excitation circuit
current due to the presence of harmonic components.
The generator does not work if there is not a certain
residual magnetism in the core. If there is no residual
magnetism, the excitation process is not initiated and
therefore in this project the residual magnetism is
generated by placing small permanent magnets in the
polar heads.
D. Brushless Excitation System
Excitation systems with DC or AC exciters and rectifiers
must be transferred from these facilities to the field
winding of the generator, demanding slips rings and
brushes. This makes both the design of these devices as
their maintenance more difficult, increasing the degree of
complexity with the increase of the nominal power of
generators. The main problems are related to cooling of
the collector rings and the useful life of the brushes. To
solve this problem successfully, an excitation system
without slip rings was projected.
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E. Rotor Winding Connection
Fig. 3 shows the connection diagram of the rotor winding.
Here, each coil consists of a half-wave rectification circuit,
connected (shorted) to a diode rectifier to form alternating
poles. These connections are made from the exterior of the
rotor winding set, allowing quick and easy implementation
to carrying out the corresponding measurements of voltage
(V) and current (A). This configuration gives the
excitation current which is the main generator field.
Fig. 3. Connection diagram of the rotor winding.
F. Prototype Construction
Fig. 4 shows part of the construction process of the
prototype brushless self-excited generator. Part (a)
presents the adaption process of the slots on the base
machine which later enabled the construction of the stator
winding (b). It can be observed the mechanic basic design
of the rotary rectifier diodes in (c) and its mounting on the
rotor in (d). Part (e) shows the stator and rotor winding
before final assembly and part (f) presents the connection
of brushless self-excitation synchronous generator on the
table test.
Fig. 4. Prototype construction process.
The asynchronous auxiliary winding is connected in
series to form 6 poles in single phase configuration. This
winding has a resistance of 33.9 Ω.
The polar heads of the inductor winding on the rotor are
placed alternately and separated by 10 slots which house
the partial side coil, where each coil consists of 60
conductors.
On the other hand, the radial length of the air gap
between rotor and stator is 2.5mm and the necessary
induction on the rotor machine is 1.15 Tesla. The
amperes-turns for meter (A-turns/m) necessary to
determine the magneto-motive force (mmf) in each
circuit of the generator were obtained through the
magnetization curve (B-H) of the machine study,
obtained through of the laboratory test and shows in the
Fig. 5.
Fig. 5. Magnetization curve (B-H) for generator study.
4. Test Results and Discussion
The mathematical model used in computer simulation
was implemented in MatLab® environment [16]. Fig. 6
illustrates the rotor winding current behavior (excitation
current of machine) before passing through the half-wave
rectifier circuit. The capacitor current IC variation,
flowing through the auxiliary stator winding it is also
shown in this figure.
Fig. 6. Temporal variation of the rotor and capacitor currents
without load connected on generator.
Fig. 7 shows the temporal variation of the magnetizing
inductance of the generator during self-excitation. It is
noted that after 0.2 second of simulation, this inductance
abruptly decreases in relationship to its initial values
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generating transient deformation on voltage wave of the
generator as shown in Fig. 8. It is noted that the voltage
wave distortion occurs periodically each 1/2 cycle.
Fig. 7. Temporal variation of magnetizing inductance during self-
excitation of the SGEA.
Fig. 8. Temporal variation of single-phase voltage of the
generator during self-excitation process with load of 100Ω.
A. Prototype Validation: 4 Pole Machine with Varying
of Excitation Voltage
Table I lists the numerical values for armature voltage as
well as voltage and current exciter obtained through tests
in laboratory. These values were obtained for 4 pole
configuration with 1500 rpm.
Table I. – Measurement armature volts for different excited
current. (4 poles, 1500rpm)
Excitation
Parameters R-phase S-phase T-phase
Vexc [V] (Iexc) [A] [V] [V] [V]
5 0.02 10 11.4 13.2
10 0.06 19.7 22 26
15 0.09 30 32.9 39.1
20 0.12 40.3 43.5 51.2
25 0.15 51.1 55 65.2
30 0.19 62.2 66.2 79.3
35 0.22 78 83.7 94.6
40 0.25 91 97 108
Fig. 9 shows a plot of the output armature voltage of the
machine vs excitation voltage having an excitation
capacitance constant of 100µF.
Fig. 9. Armature voltage vs excited voltage for proposed
machine with excitation capacitance of 100µF and without load.
Along the self-excitation process, the terminal voltage of
the generator changes from transient to steady state
reaching a peak value of 320 Volts, as shown in Fig.10.
In this figure, it can be observed the deformation caused
by the magnetizing inductance variation.
Fig. 10. Temporal variation of the terminal voltage generator.
Fig. 11 shows the temporal variation of the load current
of the SGEA to load of 250W.The waveform of this
current is similar to the waveform terminal voltage
generator, ie, has the same periodic distortions.
Fig. 11. Temporal variation of the load current to load of 250W.
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5. Conclusions
This paper presents the design, construction and basic
analysis of a synchronous generator with brushless
asynchronous excitation. The results presented are part of
a research project development in the Engineering Faculty
of the National University of Misiones, Argentina.
The brushless self-excited three-phase synchronous
generator is of simple construction, showing robust and
has a low maintenance level mainly because not having
slip brushes in the excited system.
The mathematical model used in this study was adequate,
since the simulation results were consistent with the
experimental results, representing consistently the
expected behavior of the generator. In this sense, it was
verified that the generator output voltage remained
constant for balanced electrical load variation.
Due to the variation of magnetizing inductance during the
self-excitation of the generator, fundamental components
and other harmonics are generated in the magnetic flux
and the rotating field. This results in distorted voltages and
currents in the terminal outputs of the generator.
In relation to the mechanical construction, the results
related with torque and speed generator were not presented
in this paper, however in test development a significant
vibration was observed in the structure under load
variation condition. On the other hand, steady overheating
occurred in the pole pieces leading to a consequent
reduction in the life of the machine.
The great difficulty which had to be overcome for the
development of the excitation system without brushes was
the intensity of centrifugal efforts which rectifier and
protection devices would be subject. The use of higher
frequencies for AC exciter increased the level of the
excitation voltage dramatically improving the efficiency of
the system.
Acknowledgment
The authors thank Engineering Faculty of National
University of Misiones for their cooperation in
constructing and test of study machine.
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